The present invention relates to a system for selectively inhibiting the viewing by unauthorized subscribers of designated channels in a CATV system. More particularly, the invention pertains to such a system which is usable in CATV systems employing different transmission techniques.
In most CATV systems, certain channels are designated as "pay" channels for which the subscriber must pay a special fee to be able to watch. Thus, if a subscriber is not authorized to view a particular channel, the CATV system must be provided with a viewing control system having the capability of inhibiting viewing of that channel.
Conventionally, several different viewing control systems have been known, each employing a different technique to achieve the channel inhibiting function.
In one system, known as the jamming system, a carrier having a level about the same or greater than the level of the video carrier level and at a frequency about 2.5 MHz above the center frequency of the video carrier is injected at the CATV control center into the video band of each "pay" channel. For each authorized subscriber, a notch filter is provided for each channel to be unjammed to remove the carrier. This system, however, suffers from the drawback that one filter must be provided for each unjammed channel for each subscriber. Also, the filter must have excellent characteristics since, for instance, if the center frequency of the filter is even slightly different than the frequency of the jamming carrier, a beat will occur in the viewed picture. Thus, the filters are unavoidably expensive.
In another conventional system, known as the trap system, traps to block unauthorized reception are provided at the tap-off point to the subscriber's terminal. The disadvantages of this system are that the traps must be located in a difficult to access area, usually up a utility pole. Besides being difficult to install, the traps take up a great deal of space, which is usually at a premium at the location at which the traps must be installed. Moreover, if a large number of traps are employed in the tap circuit of a subscriber, the overall quality of the subscriber's reception is degraded.
In a further conventional system, known as the addressable terminal system, a digital signal is transmitted from the CATV control center to a terminal unit in each subscriber's home or office, the digital signal indicating which channels are authorized to be viewed. In the subscriber's terminal unit, the digital signal is decoded and unauthorized channels are jammed. This system though is disadvantageous in that a costly terminal unit must be provided each television set connected to the system, even if many different televisions are connected at a single subscriber's location. Also, it is possible for a subscriber to alter the terminal unit to receive unauthorized channels without paying the required fee. Moreover, access to the subscriber's premises must be had to service the terminal unit.